In this experiment, we investigated whether book covers can signal sub-genre information to knowledgeable readers. Self-identified science-fiction fans and mystery fans sorted 80 randomly selected book covers from each of those genres into groups of their own devising. The sorts were used to identify similarity among books, and that similarity structure was used to measure similarity among subjects. Cluster analysis was then used to find groups of subjects who sorted similarly. Linear models were demonstrated that group membership was related to the knowledge subjects reported about the genres. This pattern of results supports the view that book covers constitute an implicit signaling system between publishers and experienced readers of a fictional genre.